Mac - Fix build error against mono sdk 4.8.0 alpha.
Mac - Fix build error against mono sdk 4.8.0 alpha.

* Tests against 4.6.2 and 4.8.0 alpha.

* Xamarin Studio uses mono sdk 4.6.2 whereas Visual Studio for Mac requires an alpha build of mono sdk 4.8.0.  There where some problems building against the later with glib-2.0 that were not problems with sdk 4.6.2.

* Modify the binding-gyp to probe for include header and lib of glib-2.0
   * /Library/Frameworks/Mono.framework/Versions/Current/bin/pkg-config mono-2, glib-2.0 --cflags-only-I
   * /Library/Frameworks/Mono.framework/Versions/Current/bin/pkg-config mono-2, glib-2.0 —libs

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